This invention relates in general to plasma arc torches. More specifically it relates to a method for reliably starting a transferred arc plasma torch.
Reliable ignition of a plasma arc torch has been a significant problem throughout the development of plasma technology for cutting metallic workpieces. It is particularly important today where multiple cuts are made in unison by multiple torches. All torches should start at substantially the same time; and it is essential that they all start regardless of the precise timing of the start. More generally, reliability of starting is increasingly important as plasma cutting torches are used in robotic applications where human interventions to replace or repair a torch that will not start is a serious detriment to the reliability and cost effectiveness of a large automated operation.
One solution has been contact starting, one form of which is described in commonly assigned U.S. Pat. No. 4,791,268. However, the principal starting technique in use today uses a high frequency high voltage (HFHV) signal coupled to a power line from a D.C. power supply to an electrode of the torch. The HFHV signal induces a spark discharge in a plasma gas flowing between the electrode and a nozzle, typically in a spiral path. A HFHV generator is usually incorporated in a power supply or in a "console" located remotely from the torch and connected to the torch by a lead set.
While a number of HFHV generators are known, e.g. capacitive discharge circuits and high voltage transformers, the most common type, shown in FIG. 1, is a Marconi RF generator. The generator produces, for typical plasma arc ignition purposes, a 5 to 10 kV impulse that oscillates at 1 to 3 mHz. This signal propogates through the lead set to the electrode (cathode) and nozzle (anode) where it ionizes the plasma gas to produce charge carriers. The ionized charge carriers in the plasma gas create a current carrying path that can sustain an arc.
While this technique seems straightforward in practice, it is a difficult and complex problem. At the time of arc ignition, the location of the arc on the electrodes, and its maintenance once it is initially struck depend on many factors that vary, and some of which may be interdependent. The result is that the voltage at which breakdown occurs, and the time at which it occurs, are random events. Some of the factors include the cathode and anode geometries and gap spacing, gas pressures, the type of gas, impurities in the gas, nature of local gas flow around the electrodes (laminar, turbulent, amount of swirl), the materials forming the anode and cathode and their surface condition, the place on the electrode where the arc initiates, the available voltage from the power supply, the transient response of the power supply, and electrode and nozzle wear. Randomness is a function of where the arc initiates on the electrode because the arc usually strikes well up on the body of the electrode. The arc then travels down the electrode to a hafnium or tungsten insert following the swirling path of the gas. The path and the rate at which the arc follows the path are not predictable.
Interaction of these variables further complicates an analysis or control of ignition. A change in the arc current varies the gas pressure in the torch and the gas flow rate. Electrode and nozzle wear alter the physical location of the initial arc strike, the arc path over the electrode, and the time for the arc travel. Gas impurities deposit on the electrode and nozzle; these deposits change the physical location of the arc strike and the arc voltage. In turn, any increase in the arc voltage, regardless of its source, reduces the ability of the surge injection circuit to provide an initial arc current and, once the arc is struck, to act as a current source sufficient to build to and sustain a steady-state pilot arc.
If an arc strikes, but extinguishes before it transfers, the most common general solution to date has been to attempt to restart the torch, and in particular to operate the HFHV generator constantly until the arc strikes and transfers. With a constant HFHV signal, if the arc extinguishes at any point in the ignition process, the starting circuit will automatically and immediately begin to restart the arc. This arrangement is shown in FIG. 3 and will be discussed in more detail below.
This automated restart, however, is also unreliable. At each restart the surge injection circuitry has less stored energy for the arc. The restart sequence ratchets down in energy and progressively lowers the probability of a successful ignition. In a certain percentage of cases the torch will not ignite, even with the steady application of a high voltage, high frequency signal.
It is therefore a principal object of this invention to provide a method of starting a plasma arc cutting torch with a high frequency high voltage signal that is highly reliable despite adverse variations in operating conditions.
Another object is to provide the foregoing advantage in a manner that is compatible with known starting circuits and can be readily retrofitted onto existing systems.
A still further object is to provide a system for starting multiple torches at substantially the same time with a high degree of reliability to facilitate simultaneous cutting operations.
Another object is to provide a plasma cutting torch starting system that is conducive to use with robotics.
Yet another object is to provide a system with the foregoing advantages which has a favorable cost of implementation.